Before scheduling an event group, we first check if a group can goon. We first check if the group is made of software only eventsfirst, in which case it is enough to know if the group can bescheduled in.

For that purpose, we iterate through the whole group, which iswasteful as we could do this check when we add/delete an event toa group.

So we create a group_flags field in perf event that can hostcharacteristics from a group of events, starting with a firstPERF_GROUP_SOFTWARE flag that reduces the check on the fast path.